The present invention relates to a process for preparing 3-halogeno-2-hydroxypropyltrimethylammonium halide having the formula (2): ##STR1## wherein each of X.sub.1 and X.sub.2 is the same or different halogen atom.
It is known that 3-halogeno-2-hydroxypropyltrimethylammonium halide is a useful intermediate for the synthesis of carnitine and that especially (S)-3-halogeno-2-hydroxypropyltrimethylammonium halide, having the formula (2a): ##STR2## wherein X.sub.1 and X.sub.2 are as defined above, is a useful intermediate for the synthesis of (l)-carnitine (Japanese Unexamined Patent Publication No. 231632/1985).
(l)-carnitine, which is known as Vitamin B.sub.T and exists widely in a human body, plays an important part as a carrier of a fatty acid having a long chain and also has recently been noticed as the treatment for the carnitine deficiency.
Hitherto, as a method for preparing the compound (2), there has already been known a method for preparing it by reacting epihalohydrin with trimethylammonium halide.
Also, it may be pointed out a method for preparing the compound (2a) by reacting (S)-epihalohydrin with trimethylammonium halide, but is is not easy to prepare (S)-epihalohydrin economically.
There is another method for preparing the compound (2a) from the corresponding racemate (2) by resolution (Japanese Unexamined Patent Publication No. 231632/1985), but this method requires complicated operations to obtain high optical purity of the compound (2a).
Therefore, in preparing (l)-carnitine, the simple and economical method for preparing the compound (2a) is desired.
As the result of the continuous effort of the present inventors in order to find a new method for preparing the compound (2) or (2a) without using epihalohydrin, it is found that 2,3-dihydroxypropyltrimethylammonium halide, having the formula (1): ##STR3## wherein X.sub.1 is a halogen atom, or (S)-2,3-dihydroxypropyltrimethylammonium halide, having the formula (1a): ##STR4## wherein X.sub.1 is as defined above can be converted into the corresponding compound (2) or (2a) by the reaction of selective halogenation of the primary hydroxyl group of the compound (1) or (1a) with a halogenating reagent in a high yield.
Consequently, the present invention has been completed.